Study on Postfrac Softening Mechanism of Deep Marine Shale Reservoir in South Sichuan Basin

Xiaojin, Zhou; Yonggang, Duan; XiaoPing, He; Bo, Zeng; Yi, Song

doi:https://doi.org/10.1155/2023/4433439

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Geomechanics of Deep Unconventional Oil and Gas Exploitation

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Volume 2023 | Article ID 4433439 | https://doi.org/10.1155/2023/4433439

Study on Postfrac Softening Mechanism of Deep Marine Shale Reservoir in South Sichuan Basin

Zhou Xiaojin,^1,2Duan Yonggang,¹He XiaoPing,³Zeng Bo,²and Song Yi²

Academic Editor: Peng Tan

Received17 Aug 2022

Revised09 Oct 2022

Accepted11 Oct 2022

Published27 Jan 2023

Abstract

Shale softening is an important factor affecting fracturing effect. Through the Brinell hardness measurement of deep marine shale reservoir in South Sichuan Basin, the softening mechanism of shale after fracturing is explored. Through laboratory test, the key parameters of shale Brinell hardness measurement are proposed as follows: indenter size 5 mm, loading force 613 N, 1226 N, and 2452 N, and holding . It is determined that the content of quartz and clay is the main factor controlling the Brinell hardness of dry shale samples, and the mineral composition, microstructure, and fluid system jointly affect the shale softening process. The results show that (1) the marine shale in the South Sichuan Basin softens obviously, with the Brinell hardness decrease by more than 50%, (2) microfracture propagation and clay hydration expansion accelerate the shale softening process, leading to the decreased porosity and the increased number of micro-pores, and (3) the softening process is different between middeep shale and deep shale, with the latter characterized by high initial hardness and low softening rate.

1. Introduction

China has made increasing efforts in exploration and development of shale gas, triggering the shale gas production to grow year by year—over in 2020. Clearly, shale gas has become a critical contributor to China’s energy supply [1–3]. Horizontal well-staged fracturing technology is the key in shale gas development. Through technology introduction, integration, and innovation, the principal fracturing technology for shale gas shallower than 3,500 m has been formed in China [4]; however, the technology for shale gas deeper than 3,500 m needs further research. In the Sichuan Basin, the shale gas resources deeper than 3,500 m account for more than 85%. Therefore, the large-scale and profitable development of such resources will be vital to the quality and sustainability of China’s shale gas industry.

It is generally believed that the difficulties in fracturing of deep marine shale in southern China lie in that (1) high stress causes great difficulty in fracture propagation and thus small stimulated reservoir volume (SRV); (2) large stress difference results in low fracture complexity; and (3) high closure stress causes more severe embedment and crushing of proppant and thus difficulty in propping of multilevel fractures [5–9]. Many scholars have investigated and validated the differences in hydraulic fracture propagation in shale caused by high stress and large stress difference, and such differences can be characterized through quantitative and visualized numerical simulation methods [10–14]. For optimization of propping and conductivity of multilevel fractures, a lot of efforts have been put in research on the influencing factors of proppant conductivity [15–17] and the effect of proppant embedment on fracture conductivity [18–22], but the softening mechanism of deep shale is rarely reported [23, 24].

Wong et al. Yu et al., and Du et al. analyzed the hazards of shale softening: (a) reduce the fracture conductivity, thereby reducing the natural gas production; (b) degradation of mechanical properties and reservoir fracturing property; (c) creep of shale formation leads to casing deformation; and (d) the borehole wall is unstable [25–27].

Huang et al. and Liu et al. studied the mechanical characteristics and acoustic emission characteristics of shale softened rock by means of experiments. The research shows that (a) the water-absorbing shale specimens experience longer compaction and crack closure stages than the natural shale specimen, (b) the mechanical properties of the water-absorbing shale specimens, such as uniaxial compressive strength, Young’s modulus, and peak strain, are less than those of the natural shale specimens, and (c) the apertures of the failure cracks in the water-absorbing shale specimens are greater than those in the natural shale specimens under uniaxial compression loading [28, 29].

Tong et al. measured the rock hardness after immersion in liquid, the hardness data showed that properly designed formulations could avoid the shale softening and even increase the hardness of the fracture surface (from 230 to 280 MPa) [30].

Research on the softening mechanism of deep shale is of great significance. Constructing “artificial gas reservoir” in shale through hydraulic fracturing is fundamental for shale gas development. The research on the solid-liquid reaction between downhole fluid and shale at different stages (fracturing, soaking, and flowback) can support the selection of fracturing fluid system, and proppant type, size and volume, design of pumping procedure, and optimization of postfrac soaking and flowback system. In this paper, the softening mechanism of deep shale in the South Sichuan Basin is discussed to clarify the differences in the softening mechanisms of deep and middeep shales, so as to support the optimization of fracturing parameters for deep shale.

2. Measurement of Brinell Hardness during Shale Softening

During hydraulic fracturing and postfrac soaking and flowback in shale gas wells, the solid-liquid reaction in rock occurs, and the downhole fracturing fluid gradually changes from water to low-salinity flowback fluid and then to high-salinity flowback fluid. With the solid-liquid ion exchange, the rock mechanical structure is changed to a certain extent. The research on this change can provide guidance for optimization of proppant type, proppant volume, and postfrac soaking and flowback system. Limited by equipment and experimental conditions, the basic evaluation of shale fracturing is currently completed with dry samples, and the evolution laws of structural parameters of shale in the process of solid-liquid reaction cannot be obtained. Kong et al. [31, 32] discussed hydraulic fracturing considering rock internal heterogeneity and the strength difference between rock base material and the weak layers. Among the indicators in shale evaluation, structural strength is one of the key shale physical properties, and it directly determines the damage mechanism and fracture pattern of shale. Among various indicators characterizing the structural strength of objects, the Brinell hardness is most widely used [33, 34]. Analysis of the shale Brinell hardness can help us further understand the characteristics and differences of shale structural strength in different formations and guide the customization of fracturing plan.

Currently, there is no specific standard for measuring the shale Brinell hardness. Some scholars and engineering professionals in the United States attempted to measure the Brinell hardness of cores from shale gas fields. The results show an obvious difference, and thus, the Brinell hardness is considered a key indicator for optimizing hydraulic fracturing process and parameters [35]. In China, the national standard GB/T 231.1-2018 [36] is available for measuring the Brinell hardness of metallic materials. Nevertheless, metallic materials are quite different from shale in physical properties. The former is characterized by strong plasticity and toughness, strong ductility, high pressure resistance, etc., while the latter is characterized by strong brittleness and multiple bedding structures. Obviously, this standard, especially the key contents such as sample preparation and test parameters, is not applicable to shale. In this study, the parameters for shale Brinell hardness measurement are analyzed according to the difference in physical properties of marine shale in the South Sichuan Basin.

2.1. Test Principle

Brinell hardness is an indicator of material hardness. It is measured in the Brinell hardness tester by pressing a cemented carbide ball into the metal surface under a certain load, holding the load for a certain time and finally releasing the load and measuring the indentation diameter on the surface of the object.

The standard Brinell hardness number (BHN) is calculated as follows: where is the applied force (N), is the indenter diameter (mm), and is the diameter of the shale surface dent measured after the test (mm).

2.2. Equipment and Samples

The Brinell hardness was measured with the Brinell hardness tester developed by Beijing University of Science and Technology (Figure 1), which consists of LED screen, micrometer eyepiece, indenter steel ball, and sample saddle. A loading force is electrically applied to the sample through the indenter steel ball to generate indentation at the pressure-bearing position of the sample. The indentation parameters can be measured directly through the micrometer eyepiece, and the indentation diameter and Brinell hardness are displayed on the LCD screen. The samples were taken from the marine shale cores drilled in the Longmaxi Fm in the South Sichuan Basin (Figure 2).

2.3. Test Plan

The process of shale softening after the shale interacts with the fracturing fluid under different conditions was simulated to guide the optimization of fracturing process and postfrac soaking and flowback system. A total of 35 cores were collected at different depths and layers in typical shale gas blocks of the South Sichuan Basin (Table 1). The Brinell hardness was measured with dry shale samples and then with shale samples immersed in water for 2 d, low-salinity (15,000 mg/L) fluid for 10 d, and high-salinity (30,000 mg/L) fluid for 10 d, respectively.

2.4. Test Procedure and Parameters

The test procedure includes core grinding (Figure 2), loading and holding, indentation measurement, and Brinell hardness calculation. Loading and holding and indentation measurement are the key steps to ensure the measurement accuracy. Before loading and holding, the test parameters for measuring the shale Brinell hardness were determined, including indenter size, loading force, and holding time. The tests on different indenter sizes indicate that too small indenter diameter causes breaking of the core, and too large indenter diameter leads to a large measurement error. After repeated tests, the optimal indenter size is determined as 5 mm. The tests under three loading forces (613 N, 1226 N, and 2452 N) suggest that the loading force of 1226 N is preferred. If the rock hardness is large and no obvious indentation is obtained under 1226 N, the loading force is increased to 2452 N. If the rock hardness is too low, the loading force of 613 N is selected. The test on holding time reveals that the indentation is basically stable when the loading time is longer than 10 s. Thus, the holding time is determined as 14–20 s. After loading, the indentation diameter was measured in an optical microscope along the vertical and horizontal directions of the indentation, and the Brinell hardness was calculated.

Due to the well-developed bedding and strong brittleness of shale, the samples are prone to local crushing, caving, and cracking during test, resulting in several uneven pits on the core surface. The test shows that a relatively stable indentation value can be obtained on a flat surface area of the sample. Based on experience, 5 measurement values were averaged.

In order to analyze the effects of fracturing fluid on the shale Brinell hardness, the Brinell hardness of 35 cores was measured in the dry state and after being immersed in water for 2 d, low-salinity flowback fluid for 10 d, and high-salinity flowback fluid for 10 d, respectively. The cores after being immersed in fluids are shown in Figure 3, with some cores partially stripped off. Such pores are more likely to break in layers during the Brinell hardness test. Moreover, because the measured core was intermittently crushed during test, several cracked areas appear on the core surface, as shown in Figure 4.

2.5. Result Analysis

2.5.1. Correlation of Shale Brinell Hardness

Brinell hardness is an indicator of material hardness, so it is believed to have a correlation with the rock mineral composition. Shale rock minerals mainly include clay, quartz, orthoclase, plagioclase, calcite, dolomite, and pyrite. In order to clarify the main mineral types that affect the shale Brinell hardness, the Pearson correlation coefficient was used to determine the correlation between parameters. The Pearson correlation coefficient between two variables is defined as the quotient of the covariance between the two variables and the standard deviation. where is the mathematical expectation of sample , is the covariance of sample , is the sample standard deviation, and is the average values of samples .

There are three cases of correlation: (1) when the correlation coefficient is 0, variables and have no relationship; (2) when increases (decreases) as increases (decreases), they are positively correlated, and the correlation coefficient is 0.00–1.00; and (3) when decreases (increases) as increases (decreases), they are negatively correlated, and the correlation coefficient is between -1.00 and 0.00.

In order to verify the reliability of the correlation coefficient, the significance test was conducted, i.e., the assumption is accepted or denied according to the principle “the low probability event does not occur.” The probability is expressed with , which is mostly 0.01, 0.05, and 0.10. The smaller value indicates the higher reliability.

The correlation analysis shows that the quartz, dolomite, and pyrite contents are positively correlated to the Brinell hardness, and the clay, orthoclase, plagioclase, and calcite contents are negatively correlated to the Brinell hardness (Figure 5). Moreover, the clay and quartz contents are apparently correlated to the measured shale Brinell hardness at the level of , and the orthoclase and plagioclase contents are apparently correlated to the Brinell hardness of shale immersed in the flowback fluid at the level of . The quartz mineral is mainly composed of SiO₂, and it has a hard texture. The clay mineral is primarily composed of SiO₂, Al₂O₃, and water, and it is soft compared with quartz and has the physical properties of hydration and expansion when it meets water. Thus, the measured Brinell hardness is consistent with the properties of single mineral in rock, which proves the Brinell hardness measurement method and test parameters are correct.

2.5.2. Law of Shale Softening

In Figure 6, the shale samples of Changning, Weiyuan, and Yuxi show the characteristics of continuous softening, and those of Luzhou show the stable characteristics after rapid softening. Cores with high quartz content show the characteristics of continuous softening, that is, as the shale samples are immersed in different liquids, the softening rate decreases, but it remains high in the high-salinity flowback fluid. Cores with high clay content show rapid softening in the early stage; they have a lower softening rate, with the rock properties becoming stable, when they are immersed in the high-salinity flowback fluid. During being immersed in the liquids, the shale shows “complementary” feature, i.e., the shale shows high softening rate when it is immersed in water and low softening rate when it is immersed in the flowback fluid, and vice versa. For different fluid systems, the shale has the highest softening rate when it is immersed in the water, with the maximum decrease rate of the Brinell hardness up to 17% per day. The shale immersed in the flowback fluid does not show an obvious rule. The shale immersed in the high-salinity flowback fluid even has a higher softening rate than that immersed in the low-salinity flowback fluid. In order to clarify the postfrac softening mechanism of shale, the microscopic pore-throat during shale softening was tested.

(a)

(b)

(c)

3. Microscopic Pore-Throat Test during Shale Softening

The microscopic pore-throat structure is a major factor affecting the shale hardness. Microscopic characterization of the softening process of shale can be realized by observing the microscopic pore-throat structure inside the shale immersed in the water. According to the pore size of the rock, a CT (computed tomography) industrial computed tomography system was used to observe and image the pore-throat structure inside the rock.

As shown in Figure 7, the processes of observation by the CT industrial computed tomography system mainly include scanning, projection, data reconstruction, slicing, and 3D reconstruction, and postprocessing is carried out to obtain the porosity, defect rate, etc.

The cores from Well N2 were scanned. A cube in the scanned core was acquired for further analysis. In Figure 8, light gray is the base, blue is the disconnected pores, red is the connected pores, and green is the fractured area.

In order to characterize the microscopic pore-throat change during shale softening when the shale is immersed in the fracturing fluid, the experiment was operated in three steps: (1) CT scanning of dry shale core; (2) CT scanning of core immersed in the water for 10 d; and (3) CT scanning of shale immersed in the flowback liquid for 10 d.

In Figure 9, over the immersion time, the microfractures propagate obviously. After the shale is immersed in the water for 10 d, the fractures propagate along the bedding. After the shale is immersed in the flowback liquid for 10 d, the fractures propagate both along the bedding and perpendicular to the bedding. The size of two major fractures was measured; the core was dried, immersed in water, and then immersed in the flowback fluid. The size of the 1# fracture increased from 0.061 mm to 0.171 mm and 0.200 mm, and the size of the 2# fracture increased from 0.032 mm to 0.046 mm and 0.076 mm.

(a)

(b)

(c)

Moreover, the change of porosity of the cores of Well N2 was analyzed. A cube of in the numerical model from CT scan was acquired for fine analysis. The cube is acquired through avoiding the effects of fractures. The porosity distribution of the local core is shown in Figure 10. After the shale is immersed in water for 10 d, the core porosity is in transition from a normal distribution with 10% as the median line under dry conditions to a distribution with 8% as the median line. The proportion of low porosity increases significantly. When the core is further immersed in the flowback fluid for 10 d, the porosity median line moves to about 5%, and the porosity further decreases. As the shale core is immersed in the flowback fluid, the shale hydrates and expands, and the pore volume is compressed, resulting in a continuous decrease in porosity.

In summary, propagation of microfractures and hydration and expansion of shale accelerate shale softening. Propagation of microfractures is conducive to improving the pore structure and permeability of the reservoir. Shale hydration and expansion compress the pore space and reduce the shale porosity and permeability. The original reservoir structure is an important factor affecting shale softening. Comparison shows that the middeep shale cores have good development of bedding, and they are mostly layered. The bedding of deep shale is relatively underdeveloped, and the cores are mostly massive. This is possibly the main reason that the softening rate of the middeep shale is significantly higher than that of the deep shale.

4. Conclusions

(1)The Brinell hardness is an important indicator of shale softening evaluation. Compared with the Brinell hardness measurement of metal materials, the laboratory measurement of shale Brinell hardness is significantly different in sample preparation, loading force, holding time, and indentation location, and it is affected by the shale characteristics, such as brittleness and tendency to cracking, development of weak structural planes, and strong heterogeneity. For the marine shale of Longmaxi Formation in the South Sichuan Basin, the key test parameters are recommended as indenter size 5 mm, loading force 613 N, 1226 N, and 2452 N, and holding time more than 10 s(2)For the dry shale samples, the Brinell hardness is positively correlated to the contents of hard minerals, such as quartz, dolomite, and pyrite, and is negatively correlated to the contents of soft minerals such as clay, orthoclase, plagioclase, and calcite. The quartz and clay contents are apparently correlated to the shale Brinell hardness at the level of . The Brinell hardness of deep shale is generally higher than that of middeep shale(3)Postfrac softening of shale is a continuous process and does not stop when the salinity of the downhole liquid increases. Shale softening is affected by mineral composition, microstructure, fluid system, etc. The shale with the high quartz content shows the characteristics of continuous softening, and the shale with the high clay content shows the characteristics of “initial rapid softening and late slow softening.” During shale softening, microfracture propagation and hydration and expansion of clay minerals occur at the same time, and the shale porosity continues to decrease(4)After hydraulic fracturing, the marine shale in the South Sichuan Basin has been softened significantly, with a maximum decrease rate of Brinell hardness more than 50%. Compared with the middeep shale, the deep shale has been softened in a significantly lower rate, which is possibly due to that the deep shale has the tighter physical properties, and the weak microstructure is relatively underdeveloped. It is suggested that the softening characteristics of shale should be fully considered in the fracturing plan to ensure the long-term effective propping of the multilevel fracture network

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The research was supported by the Sichuan Science and Technology, China Project (2022NSFSC1007).

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